1. Neutron Radiography
It is well known that certain nuclear isotopes, such as 1-hydrogen .sup.1 H, 6-lithium .sup.6 Li, 10-boron .sup.10 B, and others, have the property of presenting a particularly high probability of absorbing or scattering incident neutrons which have energies of roughly less than 10,000 eV. The established art of neutron radiography is a method by which such preferential absorption or scattering is exploited to enable the imaging of objects containing significant concentrations of neutron absorbing or neutron scattering nuclear isotopes. See, for example, Neutron Radiography, by H. Berger (Elsevier Publishing Co., New York, 1965).
In conventional neutron radiography, a sample to be analyzed is typically irradiated with an incident neutron beam. A neutron-sensitive detector is placed some distance behind the sample relative to the direction of incidence of the irradiating neutron beam to record the spatial pattern of neutron intensities exiting from the sample. Variation of neutron intensities as a function of position due to scattering or absorption in the sample can allow an image to be formed. Conventionally, the neutron-sensitive detector consists of a special photographic emulsion which is sensitive to neutrons, although other types of neutron-sensitive detectors have been used as well. Neutron radiography can also be used to determine the concentration and location of neutron absorbing or scattering materials in a sample.
Although neutron radiography has had wide application in the past to produce images of neutron absorbing or scattering bodies, it has been recognized that the quality of such images is not as great as might be desired. Among the more significant resolution-limiting factors that typically influence image quality in conventional neutron radiography are: (1) the divergence of the incident neutron beam; (2) the presence of high-energy neutrons and gamma rays in the incident beam; and (3) the interference of scattered neutrons.
The first resolution-limiting factor, neutron-beam divergence, generally has a substantial influence on the spatial resolution of the neutron radiographic image. Spatial resolution is by definition the ability to differentiate between two closely spaced objects. Because of the high intensity of the neutron beams which can be produced, nuclear reactors are the preferred neutron source for many neutron radiography applications. For the case of cold neutrons--that is, neutrons with an energy of less than about 0.01 eV--beam guides are used to direct neutrons from the reactor core to the experimental stations at which the neutron radiography is performed. The actual beam divergence at the experimental station for conventional reactor-based systems is determined by the critical angle of total external reflection for the material which is used to coat the inner surfaces of the neutron beam guides. The divergence of the beam exiting the guides is roughly twice the critical angle of the guide-coating material. Many reactor facilities use .sup.58 Ni as a guide coating-material because it has a particularly large critical angle for neutrons in the energy range of interest, which allows efficient neutron transport. See H. J. Prask et al., Journal of Research of the National Institute of Standards and Technology, volume 98, page 1 and following (1993). The .sup.58 Ni beam-guide-coating material leads to a divergence of roughly 16 mrad for neutrons with a wavelength of approximately 4 .ANG..
To reduce the deleterious effects which beam divergence has on spatial resolution in conventional nuclear radiography, it is usually desirable to locate the neutron sensitive detector as close as possible to the sample. However, it is not always possible to locate the detector close enough to obtain images of the spatial resolution desired. For example, in the case of determining the .sup.10 B concentration in a brain tumor in a rat, the detector must be located outside of the head of the rat and thus spaced at least some millimeters away from the tumor. Such a distance can lead to a perceptible loss of resolution of the nuclear radiographic image of the tumor. Moreover, the closer the neutron-sensitive detector is to the sample, the more the image will be degraded by the third factor noted above; namely interference from neutrons scattered from the sample. Thus in conventional neutron radiography, there is generally a trade-off between increasing spatial resolution and decreasing noise caused by scattered neutrons.
Neutron beam collimation devices are known to the art for reducing neutron beam divergence such as various slit arrangements or Soller slits. Such neutron-beam collimation devices generally work by tending to eliminate neutrons with more than a specified amount of divergence. Unfortunately, conventional collimation devices substantially reduce the intensity of the resulting collimated beam. As a consequence of the reduced neutron intensity, longer exposure times are required, which makes the formation of clear images more difficult. A need exists in neutron radiography for a method to decrease neutron beam divergence for increased spatial resolution, while minimizing the loss of collimated-beam neutron intensity.
As for the second resolution-limiting factor noted above, neutron beams typically contain substantial concentrations of high energy neutrons and gamma rays. It is generally desirable to filter out such high energy radiation, because the attenuation of the intensities of high energy neutrons and gamma rays by a sample is usually considerably less than for the lower energy neutrons and because many neutron sensitive detectors are essentially unable to discriminate between the desired lower-energy neutrons and the higher energy radiation. The presence of high energy neutrons and gamma rays in the incident neutron beam consequently tends to lead to a decrease in image contrast. Many conventional neutron radiographic systems have an essentially line-of-sight layout; that is, the neutron source, sample, and neutron-sensitive detectors are all substantially located on a straight-line axis. Filters currently used to filter the unwanted high energy radiation generally also tend to block a portion of neutrons in the desired energy range. In conventional neutron radiography, it has proven difficult to filter out efficiently unwanted high energy radiation.
Turning now to the third factor limiting image quality in neutron radiography, neutrons of the desired energy range which are scattered from the sample also degrade the radiographic image produced by the absorption or scattering of neutrons by the sample. Locating the neutron-sensitive detector further from the sample is one method to decrease the effects of radiation scattered from the sample. However, as mentioned above, as the distance from the sample to the detector increases, there is a loss of spatial resolution in the neutron radiographic image as a result of the divergence of the incident neutron beam.
Known to the art are anti-scatter grids for reducing the effects of scattered neutrons. Anti-scatter grids are typically composed of fine meshes of neutron absorbing materials. Such anti-scatter grids function by absorbing scattered neutrons which strike the absorbing material, thus producing a more collimated beam which is less sensitive to sample-detector separation. The process is also known as scatter rejection. Unfortunately, conventional anti-scatter grids tend to block a substantial portion of the desired radiation. Moreover, such anti-scatter grids are generally expensive to manufacture. There is a need in the art for a more efficient and cost effective mechanism for improving the resolution of nuclear radiographic images.
2. Multiple-Channel, Multiple-Total-External Reflection Optics
Multiple-channel, multiple-total-external reflection optics--referred to as Kumakhov optics--are known which are based on the phenomena of total-external reflection of x rays, gamma rays, and neutrons. Multiple-channel, multiple-total-external reflection optics are described in U.S. Pat. No. 5,192,869 to Kumakhov, the contents of which are incorporated herein by reference. The critical angle of total-external reflection; that is, the angle below which incident radiation is totally reflected, is dependent on the reflecting material and on the energy of the incident radiation. In general, for a given reflection material, the lower the energy of the incident radiation, the greater the critical angle. Multiple-channel, multiple-total-external reflection optical devices include a plurality of channels. That portion of radiation which is incident on interior surfaces of the channels at angles less than the critical angle will undergo successive total reflections within the channels, and in this way can be guided along the channel interiors. The channels can be curved to manipulate radiation beams in various ways. Multiple-channel, multiple-total-external-reflection optics have demonstrated the ability to guide thermal and cold neutrons efficiently. See, for example, M. A. Kumakhov and V. A. Sharov, Nature (London), volume 357, pages 390 and following (1992) and H. Chen et al., ibid., page 391 and following.
It is an object of the present invention to decrease the divergence of a neutron beam, with a minimum loss of neutron intensity. It is another object of this invention to provide a cost-effective means to filter out high energy neutrons and gamma rays from neutron beams. Yet another object of this invention is to filter out by absorption, unwanted neutrons which are scattered from an analysis sample. Another object of this invention is to improve the quality, and sensitivity of neutron radiographic images.